Catalysts are employed in the exhaust systems of automotive vehicles to convert carbon monoxide, hydrocarbons, and nitrogen oxides (NOx) produced during engine operation into more desirable gases. When the engine is operated in a stoichiometric or slightly rich air/fuel ratio, i.e., between about 14.7 and 14.4, catalysts containing precious metals like palladium and rhodium are able to efficiently convert all three gases simultaneously. Hence, such catalysts are often called "three-way" catalysts.
It is desirable, however, to operate gasoline engines under "lean-burn" conditions where the A/F ratio is greater than 14.7, generally between 19 and 27, to realize a benefit in fuel economy. Such three-way catalysts are able to convert carbon monoxide and hydrocarbons but are not efficient in the reduction of NOx during lean-burn (excess oxygen) operation. Efforts have been made in developing lean-burn catalysts in recent years. One deficiency of some of the conventional lean-burn catalysts is that they are based on zeolite materials which are less than durable at the elevated temperatures necessary for their efficient catalytic operation in the exhaust gas system. Recent efforts to solve the problem of NOx in lean-burn systems have focused on lean-NOx traps, i.e., materials which are able to absorb nitrogen oxides during lean-burn operation and then later release them when the oxygen concentration in the exhaust gas is reduced. Typical of material combinations in conventional traps are an alkaline earth metal like barium with a precious metal catalyst like platinum. European Patent Application 0613714A2 published Sep. 7th, 1994 discloses that platinum or palladium in various combinations with at least two ingredient materials of the alkali metals, alkaline earth metals, transition metals, or rare-earth metal are capable of storing or absorbing nitrogen oxides under exhaust conditions of excess oxygen.
The widely held mechanism for this absorption phenomena is that during the lean-burn operation the platinum first oxidizes NO to NO.sub.2 and the NO.sub.2 subsequently forms a nitrate complex with the other material, e.g., the barium. In the regeneration mode as during a stoichiometric or rich environment, the nitrate is thermodynamically unstable, and the stored NOx is released. NOx then catalytically reacts over the platinum with reducing species in the exhaust gas to form O.sub.2 and N.sub.2. Hence to use lean-NOx traps, a hybrid-mode engine strategy is used. The air/fuel ratio is cycled between extended periods of lean operations where the traps sorb NOx emissions, alternated with brief, fuel-richer intervals to desorb the adsorbed NOx and regenerate the lean-NOx trap.
The alkali metal and alkaline earth metals which are typically utilized for NOx sorption have, however, the serious drawback that they are readily poisoned by sulfur in the exhaust gas. Most fuels for automotive vehicles contain sulfur and when burnt, the sulfur is converted to sulfur compounds like SO.sub.2. Over time, the sulfur compounds react with these trap materials forming sulfates which will not revert back to the sorption material. These sulfates are inactive for NOx sorption. As a result, the typical NOx trap is strongly deactivated by sulfur in the fuel. We have unexpectedly found that by forming a lean-NOx trap using tungstophosphoric acid and platinum the sulfur poisoning deficiency of prior art NOx traps is significantly eliminated.
Some properties of 12-tungstophosphoric acid are discussed in the article "Activation of Nitric Oxide by Heteropoly Compounds: Structure of Nitric Oxide Linkages in Tungstophosphoric Acid with Keggin Units", N. Chen and R. T. Yang, Journal of Catalysis 157, 76-86 (1995). This article discloses that a tungstophosphoric acid sample adsorbs NO from a flue gas at relatively low temperatures and, upon rapid heating of the sample, a fraction of the NO is decomposed to N.sub.2. The present invention NOx trap comprises a precious metal along with tungstophosphoric acid to absorb NOx from gasoline engine exhaust gas during lean-burn operation, desorption taking place when the oxygen concentration is lowered. And the trap relies on reductants, e.g., hydrocarbons, present in the vehicle exhaust for the catalytic reduction of the desorbed NOx. We have found that this invention trap overcomes the deficiencies of prior art NOx traps in that it has excellent NOx trapping ability while being resistant to sulfur poisoning.